I think it really depends what you and your child wants. . . I know some moms that have signed up their kids for a bunch of classes to "get ahead" (Kumon, English, Math, Putonghua, etc.) and I don't agree with that. . . I think if we want our kids to have activity class. . . it should be to encourage them to develop other interests and not just focus on academics, but I seem to be a lone wolf in that regard with the parents I know from school.
I do sign up my son for outside activities, but it's really just to explore to see where he can develop his interest, we've tried, Socatots, Taekwondo, Arts, Science, etc, some he liked, some he did not. He has asked for Piano & swimming lessons, so we will be doing that this semester. He has asked to continue Science classes, but with his current status of just starting primary, I've asked him if we could wait and see how he adjusts.
I believe as long as the kids enjoy the classes and are not so hung up on the grades/scores and are not pressured, and they can handle the load, there is no harm in it.